Particle beam lithography equipment, such as an electron beam pattern generating machine, uses magnetic lenses, magnetic centering coils, and magnetic deflection coils to focus, position, and deflect a charged particle beam respectively. In the case of a typical electron beam pattern generating machine, for example, the charged particle is an electron beam. Magnetic lenses focus the beam and adjust its size and intensity as it passes through various apertures. Magnetic centering coils position the beam within the lenses and relative to the apertures or lenses. Magnetic deflection coils position the beam on the substrate. A special form of the magnetic lens is the stigmation lens. This is a magnetic octapole, made up of eight different lens type coils or poles, which focus the electron beam independently in the X and the Y axis. All of these coils are part of a column. They surround or are inside an evacuated chamber which contains the electron beam.
The magnetic deflection coils for the X and the Y axis are stacked together in what is referred to as a deflection stack, or simply a stack. Typically a 10-100 KV electron beam is deflected over a maximum deflection field, or scan length, of about 0.1 to 10 mm. For example, in deflecting a 50 KV electron beam, a typical current through the coils in a deflection stack along with the resistance of those coils would result in approximately 10 watts of power being applied to the deflection stack to produce a scan deflection of 1 mm. But at a current necessary to produce a scan deflection of 0.5 mm the power applied to the same deflection stack would be only 2.5 watts.
Similarly, current to the lenses is changed to vary focal length, which in turn varies the spot size and intensity of the electron beam as it passes through apertures. Current to the centering drives is changed to adjust the beam position relative to apertures or lenses within the column. As these currents are changed, power to lenses and centering coils will vary by many watts.
The temperature of a coil is, to a first approximation, a function of the power applied to the coil and the rate of heat dissipation of the coil. A change in power applied to the lenses, stigmation coils, centering coils, or deflection stack results in a temperature change within the coil and the column as a new dynamic equilibrium is reached. As the temperature of the deflection stack changes the coils within the stack change shape and position. As the temperature of the lenses, stigmation coils, and centering coils change these coils also change shape and position. These mechanical changes in shape and position result in a change in the electron beam position, size, intensity, and scan length at the substrate.
Temperature change in the material of the coils will also result in a change in the coil resistance. These resistance changes will effect the strength of the fields being generated by the coils. Changes in the field strength of any of the coils in the column, as a result of temperature change, can result in a change in the electron beam position, spot current, spot shape, and spot size. These effects are often referred to as “temperature drift” because they can take many minutes to settle out and are similar to electrostatic or charge drift.
The actual temperature at which the coils, and more generally the column, operate is generally not a problem. The problem is first, with the differences between the initial and final temperatures as a new spot size, intensity or scan deflection is selected. The second part of the problem is the rate at which the coils reach a stable temperature after these changes have been made. A resultant position change as a function of temperature change is rather small, typically on the order of fractions of a micron. However, when trying to hold accuracy of beam position to within a few nanometers, a change in the position of the beam, the scan length or the beam deflection, of hundreds or even tens of nanometers can be very serious and result in the degradation of the quality of a pattern being written by the electron beam on a substrate. The degradation of pattern quality can be so severe, when temperature driven beam drift results in only a few tens of nanometers of beam position change, that a pattern on a very expensive mask becomes useless. Likewise temperature drift can cause changes in spot current, spot size and spot shape which can render a pattern on a very expensive mask useless.
Vector particle beam machines are particularly sensitive to this problem. The amount of beam position deflection is very pattern dependent. As the pattern changes, the average deflection, and therefore the average power applied to the deflection coils, changes. This constant variation of average power applied to the deflection coils makes it very difficult to control the internal temperature of the coils and column. As a result, the response of the deflection coils to current drive depends on the history of pattern which has just been written.
Deflection stack coils of an electron beam pattern generating machine are tightly wound and typically encased inside of a solid metal cylinder. So there is little opportunity for dissipating the power which is being applied to the coils. Any path for energy to leave the coils is very slow. It has long been the practice in the industry to use fluid to control the temperature of deflection coils. Fluid, such as flourinert (a magnetically inert fluid) is flowed through heat exchange elements positioned in close proximity to the deflection stack. This forms a system consisting of the deflection stack, a reservoir of fluid at a temperature somewhat below the stack temperature, a heater in the fluid, and the heat exchange elements. Often this is thought of as cooling the deflection coils. What is really happening though is that the total power applied to the system is held constant by adjusting the power applied to the heater in the fluid to match the power applied to the coils in the deflection stack, such that the sum of those two components is approximately constant. Typically this is accomplished by using a Proportional-Integral-Differential loop (or PID loop) to control the duty cycle of a heater in the fluid loop. Temperature of the deflection stack can be used as an input in determining that duty cycle. The size of the deflection field can also be used as a kind of feed forward input in determining that duty cycle.
However, this method has significant drawbacks. The heat exchange elements (or sometimes called ‘cooling elements’) cannot be placed very close to coils in the stack. As a result the heat transfer between the coils and the heat exchange elements is still slow. Consequently, hot and cold spots build up within the coil. If there is too much difference between the amount of power supplied to the system by the coils and the amount of power supplied to the system by the heat exchange elements, the coils will still undergo local changes in temperature internally, even though constant power is being applied to the system and the system may stay at a constant global temperature. As a result, temperature drift still occurs.
Furthermore, control of the power applied to the fluid loop must be calibrated over a wide operating range. This is time consuming and often inaccurate. Even with carefully adjusted PID loops for the fluid cooling, the stack temperature can change by over one degree Celsius when changing the scan length (or average deflection) from 1 mm to 0.5 mm. It can take as long as thirty minutes for stack temperature to stabilize while a beam drift of hundreds of nanometers is occurring. While it is possible to wait for the temperature to stabilize, this results in a significant cost in throughput. Moreover, in calibrating beam size, position, linearity, length and rotation, it is often necessary to use multiple scan lengths. This can be like aiming at a moving target.
Other attempts to control the stack temperature include placing the heater in close proximity to the deflection stack, instead of upstream in the ‘cooling loop’. This was widely experimented with back in the mid 1980's. In this method the system consists of just the deflection stack and the heater. The heater is again driven at an appropriate power such that the total power applied to the system is again held constant. [Recently U.S. Pat. No. 5,382,800 was issued to cover this very old technique] In principle this is the same method as using a fluid cooling system to adjust the total power applied to the system, which had been widely in use in the industry for many years prior to the '800 patent. The difference is simply that the heater is placed next to the coils instead of being separated by a fluid. Thus heat is transferred directly from the heater to the coils instead of being transmitted through a fluid. The advantage of using this arrangement is that the power applied by the heater directly to the system can be changed more rapidly. There is also a little shorter lag time or phase shift between the time power is applied to the system and the effects of that power. The problem with this method is the same as with fluid cooling. There is still a path between the coils and the heater for the energy to flow along. This results in hot and cold spots. Control of the supplemental power has to be calibrated. This is also time consuming and approximate at best. Finally, while the time lag of seconds is somewhat shorter than a time lag of minutes, it is still very long on the scale of microseconds at which particle beam machines move the beam around.
Similarly the temperature of lens coils and centering coils can be controlled by active systems, as described above for stack cooling, including PID loops and thermal exchange mechanisms. Generally, though, temperature control of lens and centering coils is concentrated mainly in controlling the temperature of the driving electronics and amplifiers while the temperatures of the actual lenses and centering coils are not addressed. This is because the thermal time constants for these coils to come to equilibrium is generally believed to be much shorter than that of the deflection stack. Moreover, lenses and centering coils are not adjusted as often as deflection fields.
Even more generally the same problems are encountered in electrical servo motors for positioning a stage holding a substrate under the particle beam. As power to servo motors varies, the stage and substrate temperature is affected. A stage temperature change of only one degree Celsius can cause an expansion of the substrate by as much as one micron. This invention applies to building a servo motor wherein the power applied to motor coils can be controlled independently of the field generated by the coils.
Even more generally, the same problems may be encountered in many electrical coils producing magnetic fields where temperature control is critical. This invention can be applied to any coil producing a magnetic field where it is desired to control the power applied to the coil independently of the field produced by the coil. Or stated another way, this invention can be applied to any coil producing a magnetic field where it is desired to control the temperature produced by the coil independently of the field produced by the coil.
Even more generally, the same problems may be encountered in a device where a current element is placed in an externally induced magnetic field to produce a force or other linear effect. This invention can be applied to control the power applied to the current element, and therefore temperature of the device using the current element, independent of the force or other linear effect.